(a) Field of the Invention
The present invention relates to a Metal-Clad Cable which has an interlocked metal tape armor as part of a metallic sheath assembly which provides a low impedance and a reliable ground path and functions as an equipment grounding conductor. The present invention has a conductor assembly containing at least two electrically insulated conductors, where the at least two electrically insulated conductors are cabled together longitudinally into a bundle and enclosed within an overall cover, preferably nonmetallic. A bare bonding/grounding conductor is cabled externally over the overall cover of the conductor assembly with the same lay and in concert with the conductors cabled within the conductor assembly. Metal tape is helically applied to form an interlocked armor around the cabled conductor assembly and bare bonding/grounding conductor. The bare bonding/grounding conductor is generally a similar size to the insulated conductors present in the conductor assembly; however, as the gauge of the conductors increases, it is acceptable for the size difference between the bare bonding/grounding conductor and the insulated conductors to increase (collectively hereinafter, xe2x80x9cpreferably full sizexe2x80x9d). Fillers may then be cabled internally within the conductor assembly to fill in undesirable spaces in the conductor assembly. This sizing, along with the use of fillers where desired, enables the bare bonding/grounding conductor to be in intimate contact with the inner curves of the convolutions in the interlocked metal tape armor along a substantial length of the cable. The armor and bare bonding/grounding conductor act in concert to provide a metallic sheath assembly which has an ohmic resistance value equal to or lower than the ohmic resistance requirements necessary to qualify as an equipment grounding conductor. In addition, an electrically insulated grounding conductor may be cabled within the conductor assembly to provide a redundant grounding path for cable which is to be used in patient care areas of Health Care Facilities, in addition to other applications.
(b) Description of the Prior Art
Underwriter Laboratory Standard for Safety for Armored Cables UL 4 (hereinafter xe2x80x9cAC Cable,xe2x80x9d and hereinafter xe2x80x9cUL 4xe2x80x9d) provides construction guidelines and performance standards for interlocked steel or aluminum armored cables which contain two, three, or four electrically insulated conductors of size No. 14 AWG to 1 AWG. Cables covered by UL 4 are for use as type AC Cable in accordance with Section 333, and other applicable sections, of the National Electrical Code (hereinafter xe2x80x9cNECxe2x80x9d).
NEC Section 333 provides minimum requirements for type AC Cable, including construction requiring a flexible metal tape armor and an internal bonding strip of copper or aluminum in intimate contact with the armor for its entire length, in accordance with NEC Section 333-19; electrically insulated conductors with an overall moisture-resistant and fire-retardant fibrous covering, in accordance with NEC Section 333-20; and an adequate equipment grounding path, in accordance with NEC Sections 333-21 and 250-2(d).
NEC Section 250-2(d) requires that the fault current path shall be permanent and electrically continuous, capable of safely carrying the maximum fault likely to be imposed upon it, and shall have sufficiently low impedance to facilitate the operation of overcurrent devices under fault conditions.
Underwriter Laboratory Standard for Safety for Metal-Clad Cables UL 1569 (hereinafter xe2x80x9cMC Cablexe2x80x9d and hereinafter xe2x80x9cUL 1569xe2x80x9d) provides construction guidelines and performance standards for metal-clad cables which may or may not contain optical fiber members and which contain one or more electrically insulated conductors of size No. 18 AWG to 2000 kcmil. Cables may, but are not required to, contain fillers. If fillers are used, they must be cabled or laid straight with the electrically insulated conductors. Cables covered by UL 1569 are for use as type MC Cable in accordance with NEC Section 334, and other applicable sections of the NEC.
NEC Section 334 provides minimum requirements for type MC Cable, including construction requiring (1) an armor of continuous and close fitting interlocking metal tape, or (2) a smooth metallic sheath, or (3) a corrugated metallic sheath, in accordance with NEC Sections 334-1 and 334-22; solid or stranded copper conductors with a minimum conductor size of 18 AWG, or solid or stranded aluminum or copper-clad aluminum conductors with a minimum conductor size of 12 AWG, in accordance with NEC Section 334-20; and, an adequate equipment grounding path, in accordance with NEC Sections 334-23 and Article 250.
UL 1569 Tables 6.1 and 6.2 set forth the smallest acceptable sizes for copper, aluminum or copper-clad aluminum grounding conductors in cable containing 90 degree Celsius and 75 degree Celsius circuit conductors, respectively.
NEC Section 250-96 requires that the cable armor which serves as a grounding conductor must be effectively bonded where necessary to ensure electrical continuity and the capacity to conduct safely any fault current likely to be imposed thereon.
NEC Section 250-118 requires, in applicable part, that the equipment grounding conductor running with or enclosing the circuit conductors be one or more or a combination of the following: (1) a copper or other corrosion-resistant conductor which may be solid or stranded, and electrically insulated, covered, or bare; (9) armor of type AC cable as provided in NEC Section 333-21; or (11) the metallic sheath or the combined metallic sheath and grounding conductors of type MC cable.
NEC Article 517 sets forth the requirements for wiring of Health Care Facilities. NEC Section 517-13(a) and (b) set forth the requirements for wiring of patient care areas. Pursuant to 517-13, the grounding terminals of all receptacles and all noncurrent carrying conductive surfaces of fixed electric equipment likely to become energized that are subject to personal contact, operating at over 100 volts, must be grounded by an electrically insulated copper conductor. If type MC or AC cable is used, the outer metal armor or sheath of the cable must be an acceptable grounding return path and also must qualify as an equipment grounding return path in accordance with NEC Section 250-118.
Metal sheathed cables, such as AC Cable and MC Cable, are available in armor composed of steel or aluminum interlocked metal tape. The edges of the helically wrapped metal tape armor interlock to form a series of xe2x80x9cSxe2x80x9d shaped convolutions along the length of the cable. MC Cable is also available in sheaths made of smooth or corrugated metal. MC Cable having interlocked metal tape armor and AC Cable are similar in appearance but differ in internal construction, performance, permitted uses, and safety features.
AC Cable contains a conductor assembly composed of two, three, or four electrically insulated conductors which are individually enclosed in fibrous covers. The individually enclosed conductors are cabled into a bundle wherein the conductors are twisted longitudinally together with a left-handed lay in accordance with the lay requirements defined in Section 5.5 of UL 4. The conductor assembly may contain one or more grounding conductors. If a bare grounding conductor is present, it may be individually enclosed in a fibrous cover before being cabled with the other conductors or the cabling assembly must be collectively enclosed in a fibrous cover to prevent the bare conductor from contacting the armor. During the armoring process, a bonding strip is laid lengthwise along the conductor assembly. The bonding strip and the conductor assembly are fed together into an armoring machine, where interlocking metal tape is helically applied around the conductor assembly to form an armor around the cabled conductor assembly and bonding strip. The bonding strip is composed of a strip of a thin bare copper, aluminum, or copper-clad aluminum conductor which runs longitudinally along the cable in direct contact with and electrically parallel to the armor. The bonding strip in parallel with the metal-tape interlocked armor forms a permanent, electrically conductive path that will ensure electrical continuity and the capacity to conduct safely any fault current likely to be imposed thereon. The AC Cable metal sheath assembly, comprised of the metal-tape interlocked armor and the longitudinally applied bonding strip, meets the ohmic resistance requirements for equipment grounding conductors specified in UL 4, Section 15. However, the grounding capability of the metal sheath assembly in AC Cable is significantly less than that of MC Cable of equal conductor size.
MC Cable contains a conductor assembly composed of one or more electrically insulated conductors collectively encased longitudinally in an overall covering. The overall covering may be one of several constructions specifically enumerated in UL 1569 or another construction which complies with the crushing, impact, and flexibility requirements of UL 1569. The conductor assembly may contain one or more grounding conductors, but only one grounding conductor may be bare. Generally, the conductors are cabled together into a bundle by twisting the conductors longitudinally together with a left- or right-handed lay in accordance with the lay requirements defined in Section 9.2 of UL 1569. One or more fillers, composed of a non-conductive, nonmetallic material, may be cabled (or laid straight) with the electrically insulated conductors within the conductor assembly as provided in Section 10.1 of UL 1569. After the conductors are cabled, they are collectively encased longitudinally in an overall covering. The conductor assembly is then fed into an armoring machine, where metal tape is helically applied around the conductor assembly to form an interlocked armor.
The metallic sheath of smooth or corrugated tube MC Cable may be recognized as an equipment grounding conductor if it has an ohmic resistance that is equal to or lower than the value prescribed in UL 1569. If the metallic sheath of smooth or corrugated tube MC Cable does not meet the ohmic resistance requirements, a supplemental grounding conductor sized to meet the ohmic resistance requirements, in conjunction with the metallic sheath, may be placed within the conductor assembly longitudinally, encased under the overall covering, and electrically parallel with the metallic sheath. This sheath and the supplemental grounding conductor compose the metallic sheath assembly.
UL 1569 does not currently recognize either interlocked metal tape armor alone or the metallic sheath assembly of interlocked metal tape armor MC Cable as an equipment grounding conductor. However, NEC 250-118 does recognize metal armor (the metallic sheath) alone or in combination with a grounding conductor (the metallic sheath assembly) as an equipment grounding conductor. Therefore, no change in the NEC is needed to recognize this new construction of type MC Cable.
The construction of type MC Cable has provided enhanced safety features and expanded use of metal-clad cable over that of AC Cable. Addition of the overall covering over the conductor bundle and elimination of the longitudinally laid bonding strip in MC Cable greatly reduced the possibility of damage to the conductors or conductor insulation which is present in AC Cable.
Furthermore, MC Cable has improved grounding performance over AC Cable due to construction requirements of MC Cable provided by UL 1569, Section 6.1, which requires inclusion of one of the following: (1) in cables having interlocked metal strip armor, a grounding conductor which is sized to meet the ohmic resistance values for grounding conductors provided in Table 6.1 or Table 6.2, (2) a smooth or corrugated metal sheath which meets ohmic resistance requirements provided in Table 6.1 or Table 6.2, or (3) a smooth or corrugated metal sheath and a supplemental grounding conductor sized as required to meet ohmic resistance requirements provided in Table 6.1 or Table 6.2 and electrically in parallel with the metal sheath.
Applicant is aware of no prior art where a Metal Clad Cable contains a bare bonding/grounding conductor which is cabled in concert with the conductor assembly externally over the overall cover of the conductor assembly.
The present invention relates to a Metal-Clad Cable which has an interlocked metal tape armor and a metallic sheath assembly with a low impedance and a reliable ground path which functions as an equipment grounding conductor. In the preferred embodiment, a conductor assembly contains at least two electrically insulated conductors, where the at least two electrically insulated conductors are cabled together into a bundle by twisting the conductors longitudinally together and then enclosed longitudinally within a preferably nonmetallic, overall cover. A preferably full size, bare bonding/grounding conductor is cabled externally over the overall cover of the conductor assembly with the same lay and in concert with the conductors cabled within the conductor assembly. The bare bonding/grounding conductor, which is positioned on the outside of the overall cover and within the trough created by at least two conductors cabled within the conductor assembly, is in direct contact with the inner curves of the convolutions in the interlocked metal tape armor along a substantial length of the cable. The armor and the bare bonding/grounding conductor are electrically in parallel and act in concert to provide a metallic sheath assembly which has an ohmic resistance value equal to or lower than the ohmic resistance requirements necessary to qualify as an equipment grounding conductor. Furthermore, one or more electrically insulated grounding conductors may be added within the conductor assembly to provide redundant grounding paths.
The NEC sets forth special grounding requirements for conductors to be used in patient care areas of Health Care Facilities. NEC Section 517-13 requires dual, redundant ground paths for wiring used in patient care areas of Health Care Facilities, which requirements include (1) a ground path which must be a electrically insulated grounding conductor sized in accordance with Table 250-122, and (2) a ground path which may be a metal armor or sheath or metallic sheath assembly of a cable that is recognized by the NEC as an equipment grounding conductor. There is currently no construction of MC Cable with an interlocked metal tape armor recognized and permitted for use in patient care areas of Health Care Facilities, because no such construction has an armor or a metallic sheath assembly recognized as an equipment grounding conductor. The new interlocked metal tape MC Cable construction which is the subject of this invention has a metallic sheath assembly which functions as an equipment grounding conductor. One embodiment of the present invention contains at least one electrically insulated preferably full size grounding conductor included within the conductor assembly. This embodiment meets the NEC""s special grounding requirements for conductors to be used in patient care areas of Health Care Facilities. Revision of UL 1569, based upon the instant invention, is being pursued.
In AC cable, the bonding strip, which runs electrically parallel with and lengthwise along the armor, crosses the electrically insulated conductors throughout the cable at every twist. This construction creates the possibilities of damage to and failure of the conductor insulation caused by the angle of contact with the bonding strip, resulting in dielectric failure. It is an advantage of the present invention that cabling of the preferably full size bonding/grounding conductor outside the overall cover in concert with the lay of the conductors within the conductor assembly, rather than laying the bonding/grounding conductor lengthwise along the inside of the armor and across the twisted conductors, significantly reduces the potential for conductor insulation damage and dielectric failure because the bonding/grounding conductor does not cross electrically insulated conductors.
It is an object of the present invention to overcome the shortcomings of the prior art devices described above.
It is a further object of the present invention to provide a construction of interlocked metal tape MC Cable which provides grounding performance equal to the current constructions of MC Cable while reducing the material costs and weight of the cable.
It is a further object of the present invention to provide a construction of interlocked MC Cable where the metallic sheath assembly has an ohmic resistance equal to or lower than the ohmic resistance requirements necessary to qualify as an equipment grounding conductor.
It is a further object of the present invention to provide a construction of interlocked metal tape MC Cable where the metallic sheath assembly works in conjunction with an additional grounding conductor to provide dual, redundant ground paths which satisfy the NEC""s grounding requirements for use in patient care areas of Health Care Facilities, in addition to other applications.
More particularly, the preferred embodiment of the present invention comprises an interlocked Metal-Clad Cable, having: a conductor assembly having at least two electrically insulated conductors, where the at least two electrically insulated conductors are cabled together into a bundle by twisting the conductors longitudinally together and where the cabled bundle of at least two electrically insulated conductors is enclosed within a preferably nonmetallic, overall cover; one preferably full size bare bonding/grounding conductor, where the preferably full size bare bonding/grounding conductor is cabled externally over the overall cover of the conductor assembly with the same lay and in concert with the at least two electrically insulated conductors cabled within the conductor assembly, and where the preferably full size bare bonding/grounding conductor is positioned within a trough created by the at least two conductors cabled within the conductor assembly; and, a roughly tube shaped covering, where the covering is an interlocking metal tape helically applied around the conductor assembly and the bare bonding/grounding conductor to form an armor covering, and where the armor covering is in engagement with and electrically parallel to the bare bonding/grounding conductor. Additionally, at least one electrically insulated grounding conductor may be cabled together with the at least two conductors within the conductor assembly.